While it varies by species and the type of operation, the most important parameters to track are dissolved oxygen, pH, temperature, salinity, ammonia, nitrates, and nitrates. Secondary parameters such as phosphates, ORP, and TSS become more important in outdoor systems that vary depending on the time of day and how the micro-flora and fauna are interacting with the sun.
The most common handheld meter used today is the YSI Professional Plus. It's a solid piece of hardware with a multi-parameter sonde attached that is commonly used to measure: pH, temperature, conductivity, dissolved oxygen, and ammonia/ammonium. While quite expensive ($3,440), such equipment is trusted throughout the industry. They do require considerable maintenance, cleaning, and recalibration. Plus the probes do wear and need to be replaced annually.
Another set of tools that are often used are photometers. These use reagents to measure specific parameters that are more expensive to monitor with today's probes. This model can measure up to 100 different parameters. Users mix the water sample with the reagent at the device reads the returning color. The device itself is not cheap at $1,400 and the reagents are expensive as well, at $41.50 per 50 tests.
The industry is moving in the direction of online continuous monitors, but it's going slowly. The existing solutions are both very expensive and difficult to maintain at scale. These are the problems we are trying to solve with OsmoBot - an inexpensive aquaculture monitor that never needs calibration, but just an exchange of existing sensor cartridges.